1. Field of the Invention
This invention relates to electrostatic chucks for clamping semi-conductor wafers and processing apparatus and methods utilising such chucks.
2. Description of the Related Art
In the micro-fabrication of semi-conductor, micro-mechanical, optical and other devices on or from semi-conductor wafers, it is common to pattern the upper layer of or on a semi-conductor wafer by depositing a photo-sensitive coating, defining the pattern using a lithographic process and developing the resultant pattern.
The developed photo-sensitive coating then forms a mask which can either be used to protect certain areas of the wafer from a subsequent etch process or the coating can be utilised as part of a "lift off" process where a subsequent layer is deposited over the patterned material and removal of the patterned material, lifts off the subsequent layer except where there were exposed areas defined by the pattern.
For many such processes the gases or vapours that are used at low pressure have a tendency to condense or otherwise adhere to surfaces which are cooler than the gas or vapour. This is a particular problem where the platen on which the wafer is located in the processing apparatus is cooled, because the build up of such material can effect the intimate contact between the wafer and the platen.
It will be understood that as wafers are exchanged during the production process, there is inevitably a period when the upper surface of the platen is exposed and hence by-products can build up on it. If the platen is constituted by an electrostatic clamp or chuck, this can be particularly problematic because the clamping becomes unreliable resulting in poor wafer cooling and other deleterious results.
Current approaches to this problem include allowing the chuck to warm up to gas ambient temperature, the chamber being pumped out to a high vacuum or an etch process being carried out while the wafer is absent from the chamber. Whilst this latter option is effective, it also etches away the very thin dielectric layers, which exist on prior art chucks, rapidly causing a break-down of the dielectric layer and hence of the clamping. The other options tend to slow down the production cycle and/or increase the cost of the apparatus.
In existing designs the dielectric layers are kept extremely thin, e.g. 4 .mu.m-200 .mu.m, so that low voltages can be used on the chuck and thus avoid high voltages appearing on the wafer. These dielectric layers are frequently defective due to pin holes and other faults, which cause failure of the electrostatic chucks.